This invention relates in general to vehicle transmission controls and, in particular, to a transmission control for a hydrostatic transmission.
More specifically, but without restriction to the particular use which is shown and described, this invention relates to shifting mechanisms especially useful for operation with the hydrostatic transmissions of an earth moving machine for maintaining the shift lever in a predetermined position until such time as the machine operator wishes to change speed and/or direction, or stop the vehicle.
Hydrostatic transmissions are frequently utilized in earth moving machines to control the direction and speed of movement of the machine in response to movement of a shift lever controlled by the machine operator. As is known, movement of the hydrostatic transmission shift lever in a forward direction causes forward movement of the machine at progressively higher speeds, while movement of the shift lever in an opposite direction slows the vehicle down. Upon passing through a neutral or idle position, in which the transmission is disengaged, a reverse gear becomes engaged with progressively higher speeds being attained in the reverse direction as the shift lever is moved rearwardly.
During operation of the earth moving vehicle, it is desirable that once the hydrostatic transmission shift lever has been positioned to operate the vehicle in a predetermined direction at a desired speed, that the setting be maintained without requiring that the operator manually hold the shift lever in the set position. Therefore, to hold the shift lever in position for maintaining the flow of pressurized fluid through the valves which control the flow of pressurized fluid to the vehicle transmission, a friction stop mechanism may be used so that the vehicle operator does not have to continually manipulate the shift lever for it to remain in its selected position enabling the vehicle to move at the constant predetermined speed.
Stop mechanisms used for such applications have generally been of the mechanical type. In such structure one portion of the stop mechanisms is fixed to the shift lever and is moved against the friction created by a second fixed stop element against which the shift lever is held, for example, by a suitable spring. While such devices are satisfactory in holding the shift lever in a set position, such mechanical devices require a considerable amount of space and are not satisfactory for use where an adjustment of speed must be made requiring the shift lever to be moved only a slight amount from the previous position. Because the initial (static) friction encountered in moving the lever from a fixed position is much greater than the dynamic friction encountered when the lever is in actual movement, the machine operator must initially apply a larger force to begin shift lever movement, which frequently carries the shift lever beyond the desired position, requiring a number of "adjusting" movements of the shift lever before the desired "new" speed has been obtained. Consequently with such devices it is difficult to finely adjust the vehicle speed.
In an effort to overcome such problems in mechanical friction stop, the structure has been modified such as disclosed in U.S. Pat. No. 3,954,146, wherein the mechanical linkage is designed such that a variable resistance is encountered as the shift lever is increasingly displaced from the neutral or idle position to establish either maximum forward or maximum rearward driving speeds from the transmission. While such systems may be suitable for some purposes, such a variable resistance does not resolve the problems inherent in overcoming static friction when attempting to make slight modifications in the vehicle speed.